1. Field of the Invention
The present invention generally relates to a voltage controlled oscillator (VCO), and more particularly to a dual band voltage controlled oscillator using a technology of switching current source.
2. Description of the Related Art
The rapid development of the coexist operation of multi-standard wireless and mobile communication has been driving conventional RF and baseband transceivers to have integrated multi-band and multi-functional characteristics, such as the multimode wireless LAN IEEE802.11a/b/g card, the integrated Blue-Tooth and wireless LAN card, and the integrated GSM/WLAN handset. This requirement has driven the conventional single-band RF circuits, such as low-noise amplifier (LNA), bandpass filter, mixers, voltage controlled oscillators (VCOs) and power amplifier (PA), to a new design era. To provide the multibands, such as 900/1800 MHz GSM mobile communication and 2.4/5.2 GHz wireless local area network, a single circuit with at least two bands becomes an important research topic. Numbers of work have been demonstrated the different approaches on the dual-band transceiver, which can effectively reduce the power consumption.
In various radio frequency (RF) active devices, the voltage controlled oscillator is an important key component. The basic theory and different design methods of voltage controlled oscillator can be found in the book “CMOS” pressed by IEEE Press. Since the voltage controlled oscillator determine the quality and reliability of information, the design of high performance voltage controlled oscillator in single chip of communication system is an important topic.
Two types of voltage controlled oscillator are adopted for CMOS RFIC: LC tank VCO and ring VCO. Such two types of VCO have their own advantages and disadvantages. The advantages of ring VCO are small chip size and large output signal due to easy integration with phase locked loop (PLL) circuit, while the disadvantages of ring VCO are poor phase noise due to more active devices using in the circuit. The advantages of LC tank VCO are better phase noise and simpler circuit topology than the ring VCO, while its disadvantages are large chip size and unstable process. Prior art can be found in U.S. Pat. No. 6,194,975 issued to Su al., entitled “Dual band VCO with improved phase noise”. It discloses the method of improved phase noise for the dual band VCO. However, too many devices are used in this disclosure, causing a complex circuit and large power consumption.
To overcome the aforementioned problems, there is needed to provide a novel dual band VCO. Thus, in the present invention, a novel dual-band VCO using a technology of switching current source is proposed.